1. Field of the Invention
This invention relates generally to medical and surgical devices and methods and more specifically to orthopedic bone plates and bone staples suitable for internally fixating and stabilizing fractured bones.
2. Description of the Related Art
Many bony fractures require stabilization that cannot be provided by external splints or casts; internal fixation is therefore required. Bone plates and bone staples are among the most common artificial orthopedic implants, and are commonly used to stabilize and internally fixate bony fractures.
A conventional bone plate is essentially a rigid metal plate drilled with guide holes through which bone screws can be passed. Bone screws are usually inserted through the mounting holes and threaded into the bone above and below the fracture to fix the bone plate, thereby rigidly stabilizing and fixating the fracture. Often the bone plate is removed after healing has occurred (although not necessarily). A conventional bone staple is generally a simple device comprising a crossbar and two spaced apart legs or arms extending down on opposite ends of the crossbar. The arms are introduced into the bone on opposite sides of a bone fracture to hold the staple in place and help stabilize a small bone fracture.
More recently, physicians have given increasing emphasis on bone plates, staples and like devices which are capable of providing compression of the fracture as well as stabilization. However, most conventional compression plates and staples are made of metal materials having moduli of elasticity much higher than that of bone and therefore a limited ability to apply controlled amounts of compressive force to a fracture.
In particular, use of such bone plates produces a mechanical system in which the majority of the stress is borne by the plate rather than the bone, a situation sometime referred to as “stress-shielding.” This situation is deleterious even to healthy, uncompromised bone, and can seriously impair the healing process in a fractured bone. Furthermore, it is now known that a controlled compressive load should be maintained across a fracture to promote rapid healing. Conventional, static bone plates do not provide or maintain such conditions.
Some bone plates and staples include provision for introducing compression across a bone fracture. In the case of bone plates the most common methods of producing compression rely on unusual bone screws or an unusual relationship between the screw and the mounting holes. In the case of bone staples the crossbar may be designed to pull the arms inward after the staple is surgically installed and thereby provide a limited amount of compressive force across the fracture. Such methods can introduce initial compression, but the compression is difficult to maintain. Small movements of the bone can interact with the typically high-modulus metallic plate or staple, causing large fluctuations of the compressive load. Furthermore, some resorption may occur as a prelude to osteosynthetic growth, resulting in contraction of the bone in the region of the fracture and releasing the compressive force. Even small contractions will produce slack sufficient to leave the fracture without compression (because the high-modulus metal plate cannot accommodate the contraction).
Alternatives to metal materials have been explored by some, including bioabsorbable materials and synthetic composite materials. Such materials appear promising, but offer their own challenges. There are still unanswered questions concerning the biocompatibility, strength, stability, reliability, wear, and ease of manufacture and handling. Most physicians continue to prefer metal plates to structures using synthetic materials for these reasons.